This invention relates to bacteria removal by ceramic filtration.
Bacteria removal from solutions by filtration was recognized as early as the 19th century by Pasteur.
Bacteria are living organisms often composed of a single cell in the form of straight or curved rods (bacilli), spheres (cocci), or spiral structures. Their chemical composition is primarily protein and nucleic acid. Bacteria can be classified by particle sizes in the range of about 0.2 to 2.0 microns.
Microfiltration membranes are used for separation processes over a range of filtration size exclusion of generally from about 500.ANG. or 0.05 micron to about 1 to 2 microns. In the context of filtration separations over an entire spectrum of small particle separation processes, reverse osmosis extends from about 1 to 10.ANG. to 20.ANG., ultrafiltration from about 10.ANG. to 2000.ANG., microfiltration from about 500.ANG. or 0.05 micron to about 2 microns, and macroparticle filtration from about 1 to 2 microns and up.
Microfiltration can be an effective means of bacteria removal because the bacteria of interest are larger than 0.2 micron.
The membrane filters suggested for bacteria removal in early attempts were made using 0.45 micron and 0.80 micron organic membranes. Later, organic membranes of 0.22 micron pore size were introduced to filter pseudomonas-like organisms.
Microfiltration membranes concentrate particulate products and are capable of separating microemulsions. Through concentration, the solids material larger than the rate pore size of the filter is retained by the filter in a retentate while water and low molecular weight solutes including salts, alcohols, or others, pass through the membrane as a permeate. The concentration operation can be limited by a buildup is called the concentration polarization layer and results in significant resistance to filtration flow.
Prior microfiltration methods for bacteria removal from liquids were identified with organic polymer structures with pore sizes larger than ultrafiltration membranes but smaller than the macroparticle filters.
Life sciences filtration applications, including bacteria removal, typically produce a slime on the polymeric membrane, including a film layer which sets up in cross-flow ultrafiltration. Polymeric membranes are susceptible to this buildup of slime and often are limited in their method of cleanup. The polymeric membrane also can be degraded by high temperatures or concentrated corrosive chemicals, e.g., such as acids or bases which otherwise would readily clean the membrane.
Polymeric membranes have this drawback not only in cleanbility but also in initial sterilization. To deliver bacteria-free product, the filter must be initially sterilized. The membrane should be sterilizable to eliminate colony-forming bacteria on the membrane structure. Further, the polymeric materials typically cannot be sterilized with very high heat, with high pressure saturated steam, or repeated cycles of low pressure steam. The same factors attributable to polymeric membranes as drawbacks for initial cleaning also apply to regeneration of the polymeric systems.
It is an object of the present invention to provide a method for sterilizing a liquid by removing bacteria through a filter which can be chemically cleaned initially and on repetitive regeneration.
It is a further object of the present invention to provide a method for removing bacteria through a filter which can be steam sterilized initially and on repetitive regeneration.
It is a further object of the present invention to provide a filter for removing bacteria from a liquid which can be used over a long period and through numerous regeneration cycles.
It is yet another object of the present invention to provide a method for removing bacteria from a liquid through a filter having high permeability.
These and further objects of the present invention will become apparent from the detailed description which follows.